The use of these complex structures, and in particular those obtained by assembling structures of different materials, is increasingly widespread in microelectronics, for highly diverse reasons. For example, these complex structures, also known as heterostructures, reduce costs by avoiding the use of costly solid (for example silicon carbide) substrates. In the case of an SOI (silicon on insulator) substrate, for example, they also facilitate isolating components from each other, thereby increasing the integration density, limiting component consumption, increasing speed, and the like.
To be advantageous, these complex structures must be compatible, subject to minor modifications, with the standard technology steps of microelectronics, for example bonding, heat treatment, lithography, doping, implantation and epitaxial growth. Now, most of these steps necessitate large changes in temperature.
Unlike a solid substrate, which changes homogeneously with temperature, heterostructures are sensitive to changes in temperature, especially if the materials constituting them have different coefficients of thermal expansion. The change of temperature can generate high internal stresses within the heterostructure because the materials change differently with temperature. If the stresses are too high, they can damage or even destroy the complex structure. The conventional solution to avoiding such problems is to limit temperature changes to moderate levels compatible with the existing structure.
There therefore remains the problem of producing a complex structure reliably and reproducibly by assembling two different materials so that the structure is able, without risk, to withstand technology steps at higher temperatures or of longer duration than those to which the art is currently limited.
Another major problem in microelectronics is that of being able to produce good quality epitaxially grown material of a given kind on another material. In this case, the problem lies in the possible difference between the lattice parameters of the two materials, i.e. that of the substrate and that to be grown epitaxially. For example, if Si0.8Ge0.2 is to be grown epitaxially on silicon, the lattice difference between these two materials is so large that it is not possible to grow an Si0.8Ge0.2 layer of more than a few hundred angstrom units thickness before the layer relaxes and forms numerous dislocations that are very harmful to crystal quality.
It is known in the art that curving a structure modifies its lattice parameter. However, it is not immediately apparent how to exploit this fact to produce epitaxial growth of high quality. Therefore, a need exists to accurately and reliably produce an epitaxially grown substrate with a matched lattice parameter from a given unsuitable substrate without modifying the epitaxial growth system.